Conventionally, a piezoelectric generator is essentially composed of one, or several or a very large number of piezoelectric elements preferably arranged on a circular dome permitting the focusing of all the minute waves created by each elementary particle constituting the transducer or transducers (see for example the document "Ultrasonics", vol. 5, of April 1967, pages 105-112; P. P. LELE).
Another device for generating ultrasonic waves focused in one point has also been described, which device comprises a plurality of transducer elements of piezoelectric type arranged externally on a circular dome, as well as its use for therapeutical purposes, for example for producing controlled ocular lesions (see article by COLEMAN, in the American Journal of Ophthalmology 86:185-192, 1978). COLEMAN also describes the paraxial presence of a diagnosis transducer.
Experience has proved that the quality of the focusing or in other words, the volume of the focal spot, will be smaller as the filling coefficient is near to 1 and as the opening defined by the angle of the circular dome is high.
It is further known that the shape of the wave at the focal point is identical or near-identical and in any case closely dependent on the shape of the wave emitted at the level of the piezoelectric transducer. In the case of a transducer with one face in contact with the water and the other face in contact with the air (the simplest mounting possible and the most used), the generated wave, after an electric pulse, is in the form of a damped sine wave presenting positive and negative waves.
It has been found that negative waves are dangerous for the tissues because they can induce cavitation effects. (see article by COLEMAN et al. in "Ultrasound in Med. & Biol., Vol. 13, No. 2, pages 69=14 76, 1987).
The prior art has already proposed to create waves called single-pole waves (document DE U.S. Pat. No. 34 25 992).
When the transducer is subjected to a very short pulse, it delivers a series of pulses, the front face emitting straight polarity waves while the rear face emits reversed polarity waves. The resultant emitted wave is of course the sum of those waves which is therefore now positive, now negative.
The object of the solution proposed in document DE U.S. Pat. No. 34 25 992 is to "separate" such positive and negatives waves. To do this, the rear face is cut irregularly in such a way that the waves reflected by said rear face are not focused or very incorrectly focused. The result is that the ratio of the positive wave to the negative wave increases as the focal point gets closer.
It should be noted that, with said solution, the second positive wave will also be incorrectly focused since it is due to the reflection of the first wave onto the rear face of the transducer.
It is also known to adapt the piezoelectric transducer by placing on the front face a material having an impedance ranging between the transducer impedance and that of the water used as transmission medium for transmitting the ultrasonic wave to the target situated at the focal point (see Vol. I, Part A of Physical Acoustics by MASON, Academic Press).
It is found then, firstly that the signal duration is very short, and secondly that the reached maximum pressure value is much higher as can be seen when comparing FIG. 1a (emission of non-adapted ultrasonic waves) and FIG. 1b (emission of ultrasonic waves by an adapted transducer).
Therefore, with the single-polar system such as proposed in DE U.S. Pat. No. 34 25 992, as only the first wave is focused and as regretfully said wave is always weak comparatively to the second and third waves, the ratio of the focused wave between an adapted transducer and a non-adapted transducer is in the region of a factor 10, but it is easy to realize that in the case of a non-adapted transducer, of which the rear face is not irregularly cut, the system generates a negative wave of amplitude equal to the positive wave.
In other terms, even with this solution, the negative wave is still present with an intensity such that it is capable of inducing cavitation effects dangerous for the tissues situated near to either the concretion or to the particular tissues to be destroyed.